'Air laser' allows remote detection of explosives

Princeton University engineers have developed a new laser sensing technology that could allow soldiers to detect hidden bombs from a distance.

The team says this 'air laser' is much more powerful than previous tools for remote measurements of trace amounts of chemicals in the air.

"We are able to send a laser pulse out and get another pulse back from the air itself," says Richard Miles, a professor of mechanical and aerospace engineering at Princeton. "The returning beam interacts with the molecules in the air and carries their finger prints."

The new technique differs from previous remote laser-sensing methods in that the returning beam of light isn't just a reflection or scattering of the outgoing beam. It's an entirely new laser beam generated by oxygen atoms whose electrons have been excited to high energy levels.

The team uses an ultraviolet laser pulse focused on a tiny patch of air, rather as a magnifying glass focuses sunlight into a cylindrical hot spot.

Within this one-millimeter-long focal point, oxygen atoms become excited as their electrons get pumped up to high energy levels. When the pulse ends, the electrons sink back and emit infrared light.

Some of this light travels along the length of the excited cylinder region, causing more electrons to fall - and thus amplifying and organizing the light into a coherent beam aimed right back at the original laser.

Researchers plan to use a sensor to receive the returning beam and determine what contaminants it encountered on the way back.

"In general, when you want to determine if there are contaminants in the air you need to collect a sample of that air and test it," Miles said. "But with remote sensing you don't need to do that. If there's a bomb buried on the road ahead of you, you'd like to detect it by sampling the surrounding air, much like bomb-sniffing dogs can do, except from far away."

The most commonly used remote laser-sensing methods measure the scattering of a beam of light as it reflects off a distant object and returns back to a sensor. But while this technique can identify contaminants, it can't detect trace amounts and can't determine the location of the gases with much accuracy.

The returning beam is thousands of times stronger in the method developed by the Princeton researchers, which should allow them to determine not just how many contaminants are in the air but also the identity and location of those contaminants.

The stronger signal should also allow for detection of much smaller concentrations of airborne contaminants, a particular concern when trying to detect trace amounts of explosive vapors. Any chemical explosive emits various gases depending on its ingredients, but for many explosives the amount of gas is miniscule.

While the researchers are developing the underlying methods rather than deployable detectors, they envision a device that is small enough to be mounted on, for example, a tank and used to scan a roadway for bombs.

"We'd like to be able to detect contaminants that are below a few parts per billion of the air molecules," Miles said. "That's an incredibly small number of molecules to find among the huge number of benign air molecules.